Lithium ion batteries (LIBs) are widely used in consumer electronics, such as laptop computers, camcorders, cameras, and cell phones, and are now being considered for applications in electric vehicles. With pressing worldwide environmental concerns, lithium ion batteries have been actively proposed for applications in electric vehicle (EV), hybrid electric vehicle (HEV) and plug-in hybrid-electric vehicles (PHEVs).
Graphite has been routinely used as a standard anode material in commercial LIBs because of its low cost, low lithium intercalation potential, and good cycling stability. However, LIBs using graphite as anode material cannot meet the stringent requirements for these applications because formation of a passivating solid-electrolyte interphase (SEI) layer on graphitic carbon depletes lithium irreversibly from the cathode on the initial charge, and the charging rate is limited because of plating of lithium on the SEI layer at the voltage required for a fast charge. Therefore, it would be advantageous to have a high capacity anode with its Fermi energy EF below the lowest unoccupied molecular orbital (LUMO) of the electrolytes so as to avoid formation of an anode SEI layer. The standard organic Li+ liquid-carbonate electrolytes have a LUMO near 1.2 eV versus Li+/Li, and an anode EF at 1.5 eV below that of lithium may compensate the loss of cell voltage by increasing the cathode capacity, providing safe high-rate performance, and by providing a longer cell cycle and shelf life. The leading anode candidate has been Li4Ti5O12 (LTO) with an EF 1.55 eV below that of lithium as it exhibits a desirable cycle life and rate capability. However, the low storage capacity of LTO (˜160 mAh g−1) has limited its further application.
Recently, TiNb2O7 has been proposed as a high capacity anode material (J. T. Han, et al., Chemistry of Materials, 23, 2027 (2011); J. T. Han, et al., Chemistry of Materials, 23, 3404 (2011)). The theoretical capacity of TiNb2O7 is 387 mAh g−1. Practically, a reversible capacity of around 280 mAh g−1 has been obtained in a voltage range of 1.0-2.5 V, which is almost two times higher than that of LTO (˜160 mAh g−1) with similar average storage voltage. Unfortunately, an intrinsic low electronic and ionic conductivity have restricted its electrochemical performance. The electronic conductivity of materials can be effectively improved by doping or carbon coating. The characteristic diffusion time for lithium diffusion in electrode materials is τ˜L2/D, where L is the diffusion length and D is the diffusion coefficient for Li+ ion in the solid. Therefore, although reducing the particle size can dramatically reduce the Li+ ion diffusion time and improve Li insertion/extraction kinetics because of the shorter Li+ ion solid diffusion pathway, significant disadvantages remain, such as low volumetric energy density, low reversible capacities and low retention of same after numerous cycles, the weak adhesion of the nanosized materials to the current collector, lower than desirable lifetimes, and health hazards associated with use of these materials.